The accurate measurement of the concentration of chiral molecules in a solution by a polarimeter is required in many applications, such as the synthesis of organic molecules where the outcome of a reaction is a racemic mixture of both enantiomers. Enantiomers have identical physicochemical characteristics except for the rotation of linearly polarized light. Enantiomers also interact differently with biological stereospecific molecules. This difference can become crucial in the pharmaceutical industry, where in many cases only one enantiomer of a drug has the desired therapeutic activity while the other can be highly toxic. For example, one enantiomer of the notorious medication Thalidomide had the intended sedative effect, while the other was extremely teratogenic and caused severe birth defects. Therefore, the accurate and sensitive determination of the ratio of the two enantiomers is of utmost importance for drugs manufacturers. A more sensitive and accurate polarimeter may mean lower concentrations of precious reagents, faster development processes, and purer, safer drugs.
Polarimeters based on coherent (heterodyne) detection (Jacobs, S. F., (1988), Optical heterodyne (coherent) detection, Am. J. Phys., 56 (3): 235-245, and King, H. J., Chou, C., and Lu, S. T., Optical heterodyne polarimeter for measuring the chiral parameter and the circular refraction indices of optical activity, Opt. Lett., 1993, 18: 1970-1972), have the advantage of being sensitive and accurate without any moving mechanical parts, which are required by ordinary polarimeters in order to rotate polarizers or move quartz plates. They also do not rely on bulky optical components, like Faraday rotators, which can be found in modern advanced polarimeters.
The optical heterodyne is created in such polarimeters when two coherent laser beams, having slightly different optical frequencies, interfere on the face of a photodetector. The photodetector acts as a mixer, generating a current containing a signal at the difference frequency (“beat”). The amplitude of the beat signal is proportional to the multiplication of the amplitudes of the electric fields of the interfering beams and, therefore, can be used to amplify weak optical signals.
When the polarization of the two interfering coherent beams is perfectly orthogonal, no beat signal can be generated. However, if the electric field vector of at least one of the beams is rotated by an optically active substance, a beat signal can be generated at the photodetector, which is directly proportional to the sine of the rotation angle. This signal can be larger by several orders of magnitude than signals generated by ordinary polarimeters, which are proportional to the square of the sine of the rotation angle.
However, like any other measuring device, the accuracy and resolution of the heterodyne polarimeter rely on the ability to discern the contribution of optical activity from variable sources of polarization noise (i.e. anything other than optical activity allowing more light to reach the detector through the analyzer), which add to the detected coherent signal. Such sources can be, for example, the finite extinction ratio of the polarizers and depolarization of the linearly polarized laser beam by scattering. Polarization noise can also be generated by residual longitudinal electric fields (created when the wavefront of the laser beams is deformed, for example, by thermal gradients) and birefringence in the optical elements along the optical path. This birefringence can be dependent on the temperature of the elements and vary when the temperature changes. If beam splitters are used in a heterodyne polarimeter, the electric field vector of the linearly polarized light beams can rotate simply because the splitting ratio of the two orthogonal components changes. This rotation can cause an increase in the background coherent signal. The splitting ratio itself can vary when the angle of incidence of the polarized laser beams is changed by thermal and mechanical motions in the mechanical frame of the polarimeter.
The contribution of the various sources of background polarization noise to the total heterodyne signal can be assessed, in principle, by measuring a reference (blank) sample. Nevertheless, this is not always possible, especially when the background signal rapidly changes or when a parallel, but different, optical setup is needed to measure the reference sample.
U.S. Pat. No. 4,832,492 discloses a heterodyne polarimeter based on a Michelson interferometer, which employs a single laser source. No mention is made in this patent to the problem of polarization noise and background subtraction. In fact, if this polarimeter is used to measure the concentration of a chiral molecule, any such noise originating in the two arms of the interferometer can be falsely interpreted as optical activity.
U.S. Pat. Nos. 5,896,198 and 6,327,037 disclose a heterodyne polarimeter employing a two-frequency laser source and a common path interferometer. However, this polarimeter cannot distinguish between a heterodyne signal resulting from increased optical activity in the sample or a one resulting from increased depolarization by scattering along the optical path. It also does not take into account the background heterodyne signal that can be introduced simply by the practical imperfection of Zeeman lasers. Any attempt in such a polarimeter to sample the laser beam by inserting a beam splitter into the optical train is likely to introduce polarization noise, which cannot be canceled.
U.S. Pat. No. 6,188,477 discloses a polarimeter based on a self-homodyne scheme. Here too, no distinction can be made between the optical activity dependent signal and signals generated by background polarization noise, for example, due to increased depolarization caused by scattering in the cell holding the sample. This polarimeter also is likely to be sensitive to mechanical phase noise in the optical setup responsible for the phase modulation. Canceling this phase noise will necessitate the sampling of the beam before the measurement cell, introducing yet another source of polarization noise.
Recently, the inventors disclosed a polarimeter for quantitative measurement of the concentration of optically active substances in a solution by either incorporating a Mach-Zehnder interferometer into a polarimeter and addressing the problem of background subtraction (WO 2008/018079), or by providing a polarimeter based on a modified Fizeau interferometer (WO 2009/069127).